Prosperous Thief is a uncommon Creature — Human Ninja card from Foundations Jumpstart (J25), collector number 346. Current market price: $0.18, TCGplayer market prices via Scryfall.
Ninjutsu {1}{U} ({1}{U}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.) Whenever one or more Ninja or Rogue creatures you control deal combat damage to a player, create a Treasure token. (It's an artifact with "{T}, Sacrifice this token: Add one mana of any color.")